1. Field of the Invention
The present invention relates generally to the bleaching of lignocellulosic materials for use in the pulp and paper industry, and more particularly to a method and apparatus for bleaching high consistency pulp with a gaseous bleaching reagent such as ozone.
2. Related Art
The use of gaseous reagents, including chlorine dioxide and ozone, for the bleaching of lignocellulosic materials including wood pulp is well known in the art. It is further known, particularly with respect to the bleaching of high consistency wood pulp, that mechanical mixing of the pulp in the presence of the bleaching reagent is required to enhance the rate of reaction between the bleaching reagent and the pulp and to achieve uniformity of the resultant bleached pulp.
As known in the art, wood pulp is obtained from the digestion of wood chips or from repulping of recycled paper or from other sources and is commonly processed in pulp and paper mills in slurry form in water. As used herein, the term "consistency" is used to express the measured ratio of dry pulp fibers to water, or more specifically, the weight of dry pulp fibers in a given weight of pulp slurry or "pulp stock" as a percentage. Various definitions are used, such as air-dry consistency (a.d. %), or oven-dry consistency (o.d. %), or moisture-free consistency (m.f. %). The laboratory techniques for measuring these values can be found in references well known in the art, such as the TAPPI Standards Manual. Terms widely used to describe ranges of stock consistency useful in pulp and paper plants follow:
Low Consistency--below about 4-6% o.d. PA1 Medium Consistency--about 9-18% o.d. PA1 High Consistency--above about 18-20% o.d., but more commonly above about 25% o.d. PA1 supplying a high consistency pulp to a first, upstream vessel; PA1 shredding the pulp within the upstream vessel in the presence of a contacting gas including the gaseous bleaching reagent, a carrier gas, and reaction by-product gases so as to suspend the pulp in the contacting gas and to initiate reaction of the gaseous bleaching reagent with the pulp; PA1 fluffing the shredded pulp within the upstream vessel in the presence of the contacting gas so as to maintain the pulp in suspension in the contacting gas and to further react the gaseous bleaching reagent with the pulp, wherein said step of fluffing includes the steps of PA1 retaining the high consistency pulp within the upstream vessel for a predetermined time which is sufficient to consume about 75% to about 90% of a selected dose of the gaseous bleaching reagent which is required to delignify the high consistency pulp from an initial Kappa number to an intermediate Kappa number; and PA1 discharging the fluffed pulp and the contacting gas from the upstream vessel to a second, downstream vessel in which the reaction of the selected dose of the gaseous bleaching reagent with the pulp is substantially completed so as to further deliginify the high consistency pulp from the intermediate Kappa number to a final Kappa number. PA1 a substantially vertical pin/foil contactor having a gas inlet, a gas outlet, a pulp inlet, and a pulp outlet; PA1 means for supplying high consistency pulp to said pulp inlet of said contactor; PA1 means for supplying fresh bleaching gas to said gas inlet of said contactor; and PA1 a porous bed reactor having a gas inlet, a gas outlet, a pulp inlet, and a pulp outlet, wherein said gas inlet of said reactor is in fluid communication with said gas outlet of said contactor and wherein said pulp inlet of said reactor communicates with said pulp outlet of said contactor; and PA1 wherein said contactor further includes means for shredding the high consistency pulp supplied to said pulp inlet of said contactor and means for fluffing the shredded pulp.
The primary characteristic of pulp slurries which changes with consistency is the fluidity. Low consistency slurries flow like water and can easily be pumped through pipelines using normal centrifugal pumps. In contrast, medium consistency pulp slurries have a paste-like character, do not flow by gravity, and can only be pumped in pipelines by using specially designed pumps. Also in contrast, wood pulp in the high consistency range does not have a slurry-like character, but is better described as a damp, fibrous, solid mass. Upon superficial examination, high consistency wood pulp appears to be and act like a dry solid. Accordingly, high consistency wood pulp generally cannot be pumped through any great distance in pipelines because the pipe wall friction is very high, resulting in uneconomic pumping horsepower requirements. However, this characteristic is used to advantage in some prior art bleaching systems which feed high consistency pulp to a gas filled vessel through a short length of pipe in which the pulp forms a plug sufficiently impermeable to prevent loss of reaction gas in the reverse direction. High consistency wood pulp has an additional characteristic which is that it can be fluffed, in the same way that dry fibrous solids such as cotton or feathers can be fluffed, to give a light and porous mass, the inner fibers of which are accessible to a chemical reagent in gaseous form. Fluffed, high consistency pulp can be blown with air or bleaching gases through pipelines provided sufficient velocity is used to prevent the wet fibers from settling out of the gas suspension. It is understood in the art that the agitation of pulp, for the aforementioned reasons, requires the expenditure of energy and increases the pulp processing costs both with regard to the initial capital investment and with regard to equipment maintenance costs in proportion to the degree of mechanical effort expended.
One known system for bleaching high consistency pulp with chlorine dioxide includes a device commonly referred to as a fluffer/blower. The pulp is mechanically fluffed within the fluffer/blower in the presence of the chlorine dioxide and the associated carrier gas so as to form a gas-suspended mixture for transport and initiation of the bleaching reaction. The gas-suspended pulp is then transported through a conduit to the top of a reactor tower, of the porous bed type. A relatively high transport velocity is required within the conduit and accordingly the flow within the conduit is turbulent in nature, which maintains the pulp in a gas-suspended mixture and continues the reaction of the pulp with the chlorine dioxide. The pulp then enters an upper portion, commonly referred to as a cyclone, of the porous-bed reactor tower in a tangential manner, causing the gas-suspended pulp to swirl around the inner wall of the reactor tower cyclone, so as to further react the pulp with the chlorine dioxide and at the same time to separate the pulp from an excess of gas required for transport, with the excess gas being returned to the fluffer/blower. The pulp then drops onto a porous bed of fluffed pulp, within the reactor tower, which continuously moves downward through the reactor tower toward an expanded section which acts as a gas separation chamber. The total residence time of the pulp within the fluffer/blower, the transport conduit and the reactor tower cyclone (prior to the pulp dropping onto the porous bed of fluffed pulp) is approximately 5 seconds. Notwithstanding the relatively short combined pulp residence time a substantial portion of the chlorine dioxide, comprising about 60% to about 80% of a given chlorine dioxide dose, is consumed within the fluffer/blower, transport conduit and reactor tower cyclone due to the very fast reaction rate characteristics of chlorine dioxide. The chlorine dioxide and carrier gas flow downward through the porous bed at a substantially higher velocity than that at which the pulp bed moves downward through the reactor, so as to substantially complete the reaction of the chlorine dioxide with the pulp. The carrier gas then flows into a gas separation chamber within the reactor and is subsequently recycled. Although bleaching systems of this type have proven somewhat effective for the bleaching of high consistency pulp with chlorine dioxide, they are subject to the following limitations. The pulp residence time within the fluffer/blower is substantially fixed and is controlled by the fluffer speed required to achieve shredding and fluffing of the pulp. The pulp residence time within the transport conduit interconnecting the fluffer/blower and the bed reactor is also substantially fixed (without an impractical increase in conduit length) due to the transport velocity required within the conduit. Accordingly, such systems provide limited flexibility with regard to the ability to vary pulp residence time, while the pulp is agitated and maintained suspended in the gaseous bleaching reagent.
Recently, there have been many efforts to utilize ozone as the bleaching reagent for high consistency wood pulp, and other lignocellulosic materials, to avoid the use of chlorine (and the attendant environmental problems) in such bleaching processes. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have limited the development of satisfactory devices and processes for ozone bleaching of lignocellulosic materials. For instance, the inventors have determined that the previously described system for bleaching high consistency wood pulp with chlorine dioxide does not provide optimum results when bleaching with ozone, due to the aforementioned inflexibility regarding pulp residence time with the pulp in an agitated, gas-suspended state. Also, a large amount of energy is required, in addition to that expended in fluffing the pulp, to transport the gas suspension of pulp from the fluffer/blower to the top of the bed reactor.
The foregoing illustrates limitations known to exist in present wood pulp bleaching operations. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.